If you are looking for new stuff in the .NET Framework v3.5, you need to look at the new assembly System.Core.dll. NOTE: In the January CTP of Orcas you have to browse the file system in order to add a reference to it (i.e. navigate to C:\Windows\Microsoft.NET\Framework\v3.5.11209\). In future CTPs, it will be added by default to NetFx3.5 projects.

There are a whole bunch of namespaces in System.Core.dll from System and System.Diagnostics to System.IO and System.Security etc (but none named System.Core). Quite frankly, I’d prefer to see the namespaces/types move to where their friends are in mscorlib.dll, system.dll etc. I strongly doubt this will happen in the short term though given the desire to leave existing assemblies with no modifications.

In other words, System.Core.dll is your main “green” assembly, while the older ones are “red”. For more on the green and red bits in Orcas read these blog entries here and here.